invention relates generally to subsurface well apparatus, more specifically, to apparatus for perforating subsurface earth formations, and particularly to pressure actuated apparatus for firing tubing conveyed perforating apparatus.
It has become common practice in the completion of oil and gas wells to perforate the well casings and the surrounding formations to bring a well into completion. One method of providing this capability has a perforating apparatus attached to the end of a tubing string which is lowered and set in place when the perforating apparatus is opposite the formation to be produced. The perforating apparatus may then be detonated and the well placed into production through the tubing string. One example of a tubing conveyed perforating apparatus can be found in U.S Pat. No. 4,523,643, issued to J. R. McGlothen, and assigned to the assignee of the present invention, which is incorporated by reference.
The systems for firing the perforating devices have typically been either an electrical firing system or a nonelectrical percussion firing system activated by dropping a member through the tubing. Neither method has been entirely satisfactory in the past. Electrical firing systems require care in connecting and running and can be activated from stray electrical currents. In addition, electrical connections can be short-circuited by moisture. Percussion firing systems commonly require a bar, referred to as a "go devil", be dropped through the tubing string thereby impacting a percussion firing assembly. These percussion firing assemblies typically have some primary explosives in the perforating apparatus while it is affixed to the tubing string and lowered into position within the well. As a result of the deficiencies of these systems, accidental and premature firings are a possibility. In addition, in a highly deviated well the bar member may not reach the perforating gun.
In an effort to overcome some of the deficiencies of the electrical and bar firing systems, various pressure responsive firing systems have been developed. In one typical embodiment and increased pressure is applied through the tubing string to the pressure responsive firing assembly. The increased pressure forces a firing pin into contact with an explosive detonator, thereby firing the perforating device. One such system is illustrated in U.S. Pat. No. 2,304,408, issued to A. J. Holifield. Systems of this type are unsatisfactory where the perforating operation is to be conducted under conditions of pressure "underbalance", where the borehole pressure is less than formation fluid pressure. Such underbalance perforating operations have become common in the area of tubing conveyed perforating.
In an effort to overcome this last deficiency a perforating firing apparatus has been developed wherein the fluid pressure in the tubing is directed into the firing apparatus where it works against a piston which shears a shear member releasing a spring loaded firing pin. This system is exemplified in U.S. Pat. No. 4,770,246 which is also commonly assigned to the assignee of the present invention and which is also incorporated herein by reference. One problem encountered in the system disclosed therein is that the shear member is subjected to the stored energy in the spring. Additionally the shear member may be accidentally rotated during assembly, prematurely releasing the firing pin. Concomitantly, the shear member must always have a tensile strength greater than the stored energy in the spring requiring higher internal pressures to break the member.
These and other disadvantages are overcome with the present invention by providing an apparatus for firing subsurface perforating apparatus using a pressure actuated firing system for use in underbalanced pressure conditions which protects the shear member from both tension and torsion forces.